gdkglcontext-win32-wgl.c: Cleanup GL context creation

gdkglcontext-win32-wgl.c: Cleanup GL context creation

Instead of first trying to explicitly ask for a WGL 4.1 context, ask for
the WGL context version that matches what is reported via
epoxy_gl_version(), so that we get the maximum WGL version that is
supported by the graphics drivers, and make sure any WGL contexts that
are shared with this (initial) WGL context are created likewise.

We can try to do a default-bog-standard 3.2 core WGL context creation
if the need arises, but let's leave that alone for now.